Pulse oximetry is a well known technique for non-invasive measurement of oxygen saturation in the blood of a living person. Generally pulse oximeters measure changes in the color of the arterial blood caused by changed in the ratio of hemoglobin and oxyhemoglobin present. The arterial blood is distinguished from venous blood and other tissue by its pulsatility.
Conventional pulse oximeters measure light transmittance through or reflectance from the blood at two wave lengths, e.g. red and infra-red. Measurements of the pulsatile and nonpulsatile components of the red and infra-red output signals are then processed using a relationship derived form the Lambert-Beers, law to compute oxygen saturation.
Some oximeters scale the magnitudes of the resultant signals making the non-pulsatile components equal so that the ratio of the pulsatile components relates directly to oxygen saturation. U.S. Pat. No. 4,407,290 to Wilbur for Blood Constituent Measuring Device and Method discloses an oximeter which scales the analog red and infra-red output signals so that their constant components are equal and then subtracts a d.c. voltage having a magnitude equal to that of the d.c. component. This enables the signals to be compared using the Lambert-Beers relationship with a simplified computation. However, the analog scaling and subtraction can provide a source of error because of limitations of the circuit and the Lambert-Beers computation, although simplified, is still complex to calculate.
Another approach, as exemplified by the pulse oximeter disclosed in European Patent Application No. 83304949.8, computes the ratios required for the Lambert-Beers relationship. A look-up table is used to apply the relationship without actually performing the mathematical manipulations required by Lambert-Beers. Although this method reduces computation time it is still prone to error resulting from deviations between empirical and theoretical factors.